The diagram is of a carbon atom. It shows a cloud of
electrons (six in this case) orbiting a central nucleus. The nucleus is the part of the atom
within the dotted line on the diagram. The nucleus of an atom is very very small – about one
hundred million million nuclei put side by side would fit into one centimetre.
The nuclear
structure of an atom was discovered by Rutherford and others in 1911
As you know the nucleus of an atom consists
of a collection of two types of particle:
(a) protons – small positively charged
particles
(b) neutrons – slightly heavier particles than protons but with no
charge
Light nuclei such as hydrogen and carbon usually contain as many protons
as neutrons but the nuclei of heavy elements such as iron, radium or uranium contain many
more neutrons than protons. You can find a list of the structure of many nuclei in
One big problems with this model on a nucleus
is why the nucleus 'sticks' together. The protons all have positive charges and the neutrons
no charge so why doesn't the nucleus simply burst apart? The positive charges of the
protons should make them repel each other – the gravitational force between them being
much too small to overcome this electrostatic repulsion.
The explanation is that
there is another force that acts only within the nucleus and between particles such as protons
and neutrons. This is called the strong nuclear force.
Within the nucleus this force is strong enough to overcome the electrostatic repulsion
between the protons and so hold the nucleus together.
In small nuclei the strong
nuclear force from all the nucleons reaches most of the others in the nucleus but as we go on
adding protons and neutrons the balance between the strong force and the electrostatic force
becomes much finer. The longer range electrostatic force affects the whole nucleus but the
short-range strong nuclear force of any particular nucleon only affects those nucleons around
it - the rest of the nucleus is unaffected. The nucleons are not held together so tightly and
this can make the nucleus unstable.